Holographic projection device for the enlargement of a visibility region

ABSTRACT

A holographic projection apparatus and a method for magnifying a virtual visibility region, for observing a reconstructed scene with at least one light modulation device and with at least one light source having sufficiently coherent light for generating a wavefront of a scene that is coded in the light modulation device. By means of imaging the wavefront into a viewer plane, it is possible to generate the virtual visibility region for observing the reconstructed scene. The virtual visibility region has at least two virtual viewer windows. In this case, the virtual viewer windows are dimensioned such that the reconstructed scene can always be observed without tracking of the viewer windows upon movement of a viewer in the viewer plane.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/680,174, filed Nov. 19, 2012, which is a divisional of U.S.application Ser. No. 12/520,188, filed Nov. 24, 2009, which claims thepriority of PCT/EP2007/063801, filed on Dec. 12, 2007, which claimspriority to German Application No. 10 2006 062 413.0, filed Dec. 21,2006, all of which are hereby incorporated in total by reference.

FIELD OF THE INVENTION

The claimed invention relates to a holographic projection device for theenlargement of a visibility region for watching a reconstructed scene,said device comprising at least one light modulator device and at leastone light source which emits sufficiently coherent light so as togenerate a wave front of a scene which is encoded on the light modulatordevice. This invention further relates to a method for the enlargementof a visibility region with the holographic projection device.

BACKGROUND OF THE INVENTION

If most displays are looked at, the observer can only see a flat pixelplane. However, it is desired to look at a display and to see theobjects presented on it in a three-dimensional view that is in arealistic manner. Various attempts have been made in this respect inorder to find a satisfactory solution for this problem. One solution isvolumetric displays, which can generate a three-dimensional image, butwhich require a complicated apparatus. Further, autostereoscopicdisplays with lenticular arrays are known from prior art documents.However, these display devices do not represent a true three-dimensionalimage. At present, the best solution of generating a three-dimensionalimage is to take advantage of holography. Holography provides a truethree-dimensional image with a desired depth accommodation (movementparallax etc.) and high resolution.

EP 1 467 263 A1, for example, discloses a holographic display device forreconstructing a three-dimensional scene. That display device comprisesa reflective light modulator, a beam splitter for projecting a hologram,an aperture stop, a field lens and a collimator lens. The hologram isgenerated on a computer using three-dimensional object information andthen presented on the light modulator. The light modulator isilluminated with light emitted by a light source and projected throughthe beam splitter, such that a three-dimensional scene is reconstructed.The reconstructed scene is generated near the field lens. However, thedimension of the scene is adversely limited by the size of the fieldlens. Further, the observer of the reconstructed scene only has limitedfreedom of movement, because a tracking facility for an observer eye hasnot been proposed. Moreover, periodical repetitions of the diffractionorders appear in the Fourier plane.

Spatial light modulators (SLM) used accordingly modulate the phaseand/or amplitude of light. Generally, such a light modulator has morethan one million modulation elements, which are referred to as pixels.In order to achieve a high resolution, and thus to get a largevisibility region and a large reconstructed scene, the light modulatoris desired to have a large number of pixels. Because the trend towardsminiaturisation is progressing at a fast pace, the light modulators areconstantly required to become smaller. As the pixel size can hardly bereduced further, however, it has hitherto not been possible from atechnological point of view, or it is at least very difficult, toachieve a large number of pixels on the light modulator with discretelycontrollable optical properties.

One possibility of enlarging the visibility region is described in“Electro-holographic display using 15 mega pixels LCD”, Proc. of SPIEVolume 2652, pp. 15 to 23, K. Maeno et al. In that display, five lightmodulators are horizontally arranged, whereby the total pixel number ofthe display becomes about 15 million pixels. Such a high pixel number ofthe display allows the reconstructed scene and the visibility region tobe enlarged, so that an observer can watch the reconstructed scene withboth eyes, i.e. binocularly. However, the disadvantage of such a displayis that the light modulators, or more precisely the light emitted by thelight modulators exhibits mutual coherence, so that the coherent lightis superimposed and disturbing interference effects such as specklesoccur. Further, it is difficult to make such a display, because themultiple mirrors needed for beam guidance must be precisely aligned.

Another way of enlarging the visibility region is described in the yetunpublished document DE 10 2006 024 356.0. The projection devicedisclosed therein comprises a two-dimensional light modulator device ina scanning system, where the light scans one after another multipleone-dimensional pixel arrangements of the light modulator device withthe help of a scanning element. A wave front modulated with the help ofthe light modulator device is imaged into a virtual visibility region oron to a screen. In order for the observer to be able to watch thereconstructed scene even if he moves, it is necessary to track thevirtual visibility region to the respective observer eye. Although it isgenerally possible to watch the reconstructed scene binocularly, this isdifficult to achieve.

SUMMARY OF THE INVENTION

It is thus an object of the present invention to provide a holographicdevice and a method which prevail over the afore-mentioned disadvantagesof the prior art, and which allow two- and/or three-dimensional scenesto be watched through a large visibility region, in particular if one ormultiple observers move, without the need to track the respectiveobserver windows. Further, DE 10 2006 024 356.0 shall be improved suchthat multiple observers can simultaneously watch a reconstructed scene.

According to the claimed invention, the virtual visibility region forwatching the reconstructed scene can be generated by way of imaging thewave front into an observer plane, whereby the virtual visibility regionhas at least two virtual observer windows which are dimensioned suchthat the reconstructed scene can always be watched without the need totrack the observer windows if an observer moves in the observer plane.

The holographic projection device comprises at least one light sourcewhich emits sufficiently coherent light, and at least one lightmodulator device. In this document, the term ‘sufficiently coherentlight’ denotes light which is capable of generating interference for thereconstruction of a three-dimensional scene. The light modulator devicecomprises pixels (modulation elements), in which the scene to bereconstructed is encoded. A virtual visibility region is generated in anobserver plane for an observer to be able to watch the reconstructedscene. The ‘virtual visibility region’ shall be construed in the contextof the present invention to be a virtual window area which is generatedto be large enough for an observer to watch the reconstructed scenebinocularly. This is achieved in that the virtual visibility regionexhibits multiple—i.e. at least two-observer windows which preferablylie side by side, and which are dimensioned such that one observer canwatch the reconstructed scene without the need to track the observerwindows if he moves in the observer plane in a region which is possiblydefined.

A holographic projection device is thus provided for a simple and quickreconstruction of two- and three-dimensional scenes in a reconstructionvolume which is as large as possible. An observer can thus watch thereconstructed scene binocularly through the visibility region, becausethe visibility region comprises multiple observer windows through whicha reconstructed scene is visible. This multitude of observer windowsalways allows the reconstructed scene to be viewed binocularly withoutthe need to track the observer window for the respective eye to a newobserver eye position if the observer moves in the observer plane. Theneed to track the observer window is thus greatly minimised or eveneliminated. Tracking means can thus be omitted in the presentholographic projection device according to this invention, which makesthe design of the projection device much simpler and compact.

According to a preferred embodiment of the invention, at least onedeflection element is provided for the generation of the virtualvisibility region with at least two observer windows. The deflectionelement makes it possible to generate a large visibility region whichcomprises multiple observer windows, in particular in the horizontaldirection, which is the coherent direction here. However, if it isnecessary to enlarge the visibility region likewise also in the verticaldirection, then a deflection element will be necessary which is able todeflect the light in both the horizontal and vertical directions, suchas for example an x-y galvanometer.

It can be particularly preferable if the visibility region can bereproduced using at least one beam splitter element in order to enablemultiple observers to watch the reconstructed scene. This is preferablymade possible by a holographic projection device for multiple observers,e.g. at events, in cinemas etc., without the need to detect eachindividual observer by a position detection system and to track therespective observer windows when individual observers move. This way,the observer can move freely within his large visibility region. Thissubstantially simplifies the holographic projection device and thecorresponding method for the reconstruction of a scene and for thepresentation of the same to the observer(s).

According to a further embodiment of the invention, if multiple lightsources are used, their light is preferably mutually non-coherent. Inparticular if multiple light sources are used for multiple lightmodulator devices, it is desired that the light of those light sourcesis mutually non-coherent, because the light is then only superimposed inits intensity, thus minimising or preventing the occurrence ofdisturbing interference effects (speckles). This substantially improvesthe quality of the reconstructed scene.

An object of the present invention is further solved by a method for theenlargement of a virtual visibility region for watching a reconstructedscene, where at least one light source emits sufficiently coherentlight, and where the light is modulated by at least one light modulatordevice, and where the modulated light is then projected through at leastone imaging element on to at least one deflection element, whereby thevirtual visibility region is generated in a predefined position in atleast one observer plane with the help of the modulated light, where atleast two observer windows are generated in the virtual visibilityregion using a multiplexing method.

The light which is modulated by the at least one light modulator deviceis imaged in at least one observer plane and generates a virtualvisibility region there. This visibility region is enlarged with thehelp of a multiplexing method by generating observer windows which arestrung together, in order to enable an observer to watch a reconstructedscene binocularly in a reconstruction volume. The visibility region canbe made large enough for the observer to be able to continue watchingthe reconstructed scene or reconstructed scenes even if he changes hisposition. It is thus no longer necessary to track the observer window,as known from document DE 10 2006 024 356.0. The method for thereconstruction of scenes is substantially simplified preferably if thespatial division multiplexing method is used, thus making possible atrue real-time representation of a moving two- and/or three-dimensionalscene.

In a preferred embodiment of the invention, the at least two observerwindows in the virtual visibility region can be generated using a timedivision multiplexing method. This is particularly advantageous if theat least one light modulator device is capable of generating the virtualobserver windows in the virtual visibility region at a very high speedand if their resolution is sufficiently high. The number of lightmodulator devices for the generation of the observer windows can thus bereduced to few modulators, or even to just one modulator. Thissimplifies the design of the holographic projection device. If multiplelight modulator devices are used, it is of course also possible togenerate the virtual observer windows using a spatial divisionmultiplexing method.

It can be particularly preferable if a Fourier transform of the lightemitted by the light source and modulated by the at least one lightmodulator device is imaged on to an imaging element which serves as ascreen, where at least this imaging element images a wave frontmodulated with the help of the light modulator device into the virtualvisibility region. Due to the discrete recording and because of theeffects of diffraction, the reconstruction of a computer-generatedhologram (CGH) is only possible within one periodicity interval of thediffraction spectrum, which is defined by the resolution of theCGH-carrying medium, namely the light modulator device. Thereconstruction is typically repeated showing irregularities in adjacentperiodicity intervals. The problem of periodic continuations of thediffraction orders of the Fourier transform is solved here by imagingthe Fourier transform on to the screen. The reproduction scale and thesize of the screen can be chosen such that the periodic continuation ofdiffraction orders lies outside the screen. As a consequence, only oneperiod is represented on the screen. This means that an observer of thereconstructed scene does not perceive the periodic continuation of thereconstruction in the diffraction orders. This greatly improves thereconstruction quality of the holographic projection device. If the wavefront was imaged into the observer plane as the Fourier transform of ahologram encoded on the light modulator device, the periodiccontinuation in the visibility region would disturb the observer inwatching the reconstructed scene because multiple observer windows wouldbe generated.

Further embodiments of the invention are defined by the other dependentclaims. Embodiments of the present invention will be explained in detailbelow and illustrated in conjunction with the accompanying drawings. Theprinciple of the invention will be explained based on a holographicreconstruction with monochromatic light. However, it will appear tothose skilled in the art that this invention may as well be applied tocolour holographic reconstructions, as indicated in the description ofthe individual embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the principles of a scanning system known from document DE10 2006 024 356.0 and illustrates the direction of propagation of thewave fronts and the direction of light propagation (coherent direction).

FIG. 2 shows schematically a holographic projection device known fromdocument

DE 10 2006 024 356.0 with the scanning system according to FIG. 1 and aprojection system.

FIG. 3 is a top view which shows schematically the scanning systemaccording to FIG. 1, where multiple light sources are provided.

FIG. 4 is a top view which shows schematically the scanning systemaccording to FIG. 2, illustrating another possibility of arranging thelight sources.

FIG. 5 shows schematically a visibility region in an observer plane.

FIG. 6 shows schematically visibility regions for multiple observers inat least one observer plane.

DETAILED DESCRIPTION

Now, the design of the holographic projection device according to thisinvention and the reconstruction of a scene, preferably athree-dimensional scene, will be described.

FIG. 1 only shows a sub-system of the holographic projection device,where a non-folded optical path is shown through deflection elements.This sub-system is referred to as the scanning system AS. It comprisesan illumination device 1 with at least one light source 2, a scanningelement 3, at least one light modulator device 4 and imaging elements 5,6, 7, 8, and 9, where each imaging element 6, 7, and 9 can consist ofmultiple single optical elements. The imaging elements 5, 6, 7, 8, and 9can be lenses, in particular cylindrical lenses, spherical lenses,corrected cylindrical or spherical lenses, diffractive optical elements(DOE), Fresnel lenses, but also mirrors or arrangements of several suchelements. Further, the imaging elements 5, 6, 7, 8, and 9 can also bedisposed off-axis, which helps to reduce aberrations, such as fieldcurvature. The holographic projection device is anamorphic, i.e. theprojection systems and thus the degrees of magnification in twodirections of the projection device which are perpendicular to eachother are different. FIG. 1 shows the scanning system AS in thedirection of propagation of the wave fronts and in the direction oflight propagation. This direction will be referred to below as thecoherent direction. The scanning system AS in the scanning direction andin the direction of light propagation, which is referred to as thenon-coherent direction, is not shown in this embodiment, because it canbe seen in DE 10 2006 024 356.0 and because it is not relevant for thepresent invention.

Now, the functional principle of the scanning system AS in the coherentdirection will be described with reference to FIG. 1. It is advantageousif diffractive optical elements are used as imaging elements, becausetheir design and functional principle are better suited to achieveaberration correction. The light source 2 of the illumination device 1emits sufficiently coherent light, which is directed on to the imagingelement 5 for broadening. The plane wave W then passes through theimaging element 6, which is for example a cylindrical lens, and isfocussed on to the scanning element 3. The scanning element 3 is thenimaged by the imaging element 7 into a plane 10 such that the wave W,which is focussed on the scanning element 3, is also focussed in thisplane 10. It is thereby possible to dispose a deflection element, suchas a mirror, in plane 10 in order to fold the optical path. If areflective light modulator device 4 is used, the deflection element isadvantageous to prevent the light on the way to the light modulatordevice 3 from overlapping with the light on the way back. Also if atransmissive light modulator device 4 is used, such a deflection elementmay be used for reasons of compactness. After having passed plane 10,the wave W is projected by the imaging element 8 as a collimated wave orplane wave on to the light modulator device 4. The light modulatordevice 4 is a reflective light modulator device in this embodiment andin the embodiments described below, in particular those according toFIGS. 1, 2 and 3, which means that there is a folded optical path, sothat a wave W with a preferably plane wave front is reflected as amodulated wave with a wave front WF. The wave front WF thus modulated bythe light modulator device 4 is then imaged by the imaging elements 8′and 9 into a plane 11. In the case a reflective light modulator device 4is used, as in this embodiment, the imaging element 8 also serves as theimaging element 8′. The two imaging elements 8 and 8′, which are shownas separate imaging elements here, are in this case combined so as toform one single imaging element. However, this only applies toreflective light modulator devices 4. While the modulated wave front WFis imaged by the imaging element 8′ (=8), a Fourier transform FT isgenerated at the same time in a plane 10′. If a reflective lightmodulator device 4 is used, the planes 10 and 10′ coincide so as to formone and the same plane. The wave front WF is then directed by theimaging element 9 on to the plane 11 as a collimated bundle of rays.Because this embodiment and any other embodiments described below areonly shown schematically, it must be noted that the imaging elements 8and 8′ are represented by one single imaging element, so that themodulated wave front WF is reflected back into plane 10′. Planes 10 and10′ thus coincide.

A description of the functional principle of the scanning system AS inthe non-coherent direction will be omitted here, because this isexplained clearly in document DE 10 2006 024 356.0.

FIG. 2 shows the holographic projection device as a whole. Theholographic projection device comprises the above-mentioned scanningsystem AS, only shown schematically here, and a projection system PS.The projection system PS comprises an imaging element 12 which serves asa screen, and at least one further imaging element 13. The imagingelement 12, which will be referred to as a screen below, can for examplebe a mirror, lens or diffractive optical element (DOE). The screen 12 isdisposed in the second focal plane of the imaging element 13. Theimaging element 13 can be a lens, DOE, lens arrangement or similaroptical element, in particular an arrangement of spherical andcylindrical lenses, so that preferably different degrees ofmagnification will be achieved in the coherent and in the non-coherentdirection. The projection system PS is further coupled with a deflectionelement 14, which is provided for the enlargement of a virtualvisibility region in an observer plane 15. The deflection element 14 isdisposed between two imaging elements 16 and 17, which form an afocalsystem. The deflection element 14 is of an individually controllabletype, preferably a mirror element and can for example be a galvanometerscanner, an array of MEMS (micro electro-mechanical systems), a polygonscanner or an acousto-optic arrangement. Further, the deflection element14 can deflect in at least one direction (horizontal and/or vertical).The wave front, which carries the information required to reconstruct athree-dimensional scene, is generated in the scanning system AS, asdescribed under FIG. 1. This is why only the reconstruction in theprojection system PS will be described with reference to FIG. 2. Theafocal system represented by the imaging elements 16 and 17 images theplane 11 through the deflection element 14 into a plane 18. Referring toFIG. 2, this plane 18 is then imaged by the imaging element 13 into aplane 19 as a virtual image, which is then imaged by the screen 12 intothe observer plane 15, where it generates a virtual observer window 21which is intended for one eye, said observer window 21 thus representinga virtual visibility region 20. At the same time, the deflection element14, which is disposed in the focal plane of the imaging element 16, isimaged by the imaging elements 17 and 13 on to the screen 12. In thecoherent direction, the modulated wave front WF is thus imaged intoplane 11 in the scanning system AS, and then into the virtual observerwindow 21 and on to the eye of an observer. The Fourier transform FT ofthe modulated wave front WF is at the same time imaged by the imagingelement 16 on to the deflection element 14. In the non-coherentdirection (not shown), the scanning element 3 is imaged into plane 11 inthe scanning system AS, and the beams are imaged into infinity orcollimated once they have passed the imaging element 9. The scanningelement 3 is then imaged into the virtual observer window 21. At thesame time, the beams are focussed on the deflection element 14 andimaged by the imaging elements 17 and 13 on to the screen.

The imaging elements 17 and 13 can also be combined so as to form onesingle lens or a lens array. The above-mentioned holographic projectiondevice was only illustrated and described for one observer eye; however,it can also be designed for a pair of observer eyes if the virtualobserver window 21 is sufficiently large, which is difficult to achievethough. Alternatively, a pair of observer eyes can preferably be servedby a second light modulator device 4 for a second observer eye; then anumber of modifications must be applied to the projection device. If theobserver is situated in the observer plane 15 and looks through thevirtual visibility region 20, here in particular through the virtualobserver window 21, he can watch the reconstructed three-dimensionalscene in the reconstruction volume 22, where the scene is reconstructedin front of, on or behind the screen 12, seen in the direction of lightpropagation.

Colour reconstruction of the three-dimensional scene is also possiblewith the help of the holographic projection device. For this, at leastone beam splitter element 23, preferably an X prism containing dichroiclayers, is disposed in front of the deflection element 14, seen in thedirection of light propagation. Alternatively, the beam splitter element23 can be disposed at any other suitable position in the holographicprojection device. Colour reconstruction of the scene is achieved bysimultaneously processing the three primary colours, RGB. If two fullyseparated light channels are provided, two beam splitter elements 23,i.e. one beam splitter element 23 per light channel, can be disposed inthe projection system PS.

It is understood that a sequential colour reconstruction of the scene isof course possible as well. To perform this type of reconstruction, apreferably coloured light source 2, which exhibits sufficient coherence,and a switching unit are required in order to control the individualmonochromatic primary colours, RGB, sequentially. This allows the colourreconstructions to be generated one after another.

FIGS. 1 and 2 illustrate the above-described holographic projectiondevice while an observer who does not move in the observer plane 15watches the reconstructed scene. However, if the observer moves toanother position in the observer plane 15, he will no longer be able towatch the reconstructed three-dimensional scene without the virtualvisibility region 20 or, in FIG. 2, the virtual observer window 21 to betracked accordingly. The deflection element 14 can then serve to trackthe observer window 21. However, such tracking requires additionaloptical devices, such as for example a position detection system, whichdetects the positions of the observer eyes in the observer plane 15.

One possibility of watching the reconstructed three-dimensional scenewithout the need to track the observer window 21 in the observer plane15, when the observer changes his position, is to enlarge the visibilityregion 20. FIGS. 3 and 4 illustrate options which can be used to achievethis.

FIG. 3 shows a scanning system AS according to FIG. 1, where theillumination device 1 comprises multiple light sources 2, here threelight sources 2. The light sources 2 are arranged parallel to each otherand parallel to an optical axis OA, and they should preferably benon-coherent with respect to each other for the same visibility region20. This can be ensured for example by using different light sources,e.g. different lasers. This is particularly advantageous insofar as thelight is then only to be superimposed as regards its intensity, but doesnot show interference. Disturbing interference effects, such asspeckles, which would substantially impair the quality of the scene, canthus not occur. It is nevertheless of course also possible to use lightsources 2 which exhibit mutual coherence. However, the hologram whichcarries the information of a scene to be reconstructed should then bedivided up into multiple light modulator devices 4. Design and workingprinciple of the scanning system AS are generally identical to thoseexplained regarding FIG. 1. However, at least two light modulatordevices 4, in this embodiment three light modulator devices 4, areprovided for the enlargement of the visibility region 20 and arranged inthe scanning system AS. The light modulator devices 4 are of areflective type, and the number of the light sources 2 used is identicalto the number of light modulator devices 4 for a monochromaticreconstruction. In order to combine the individual waves W, which areemitted by the light sources 2, after broadening by the respectiveimaging element 5 or 6, a large imaging element L is provided withcollimated optical paths. Using this imaging element, the individualwaves W can be focussed on the scanning element 3, so that all lightmodulator devices 4 are scanned simultaneously. Because the lightmodulator devices 4 must be arranged at an angle to the optical axis OAfor this, it is necessary for each light channel to comprise behind theplane 10 an imaging element 8, so that collimated light falls on therespective light modulator device 4 in order to scan the same. Thedesign of the projection system PS, which is disposed behind thescanning system AS, seen in the direction of light propagation, isidentical to that described regarding FIG. 2. However, one deflectionelement 14 must be provided per light modulator device 4. Multiplevirtual observer windows 21 are generated in this embodiment using amultiplexing method, here using a spatial division multiplexing method.Multiple virtual observer windows 21 of any size are thus simultaneouslygenerated in the virtual visibility region 20. This means that thevirtual observer window 21 can be made large enough to cover both eyesof an observer. However, it is also possible that the virtual observerwindow 21 is so small that two virtual observer windows 21 are requiredto cover one eye pupil. Further, there can be free space between twovirtual observer windows 21 which are arranged beside one another. Theentire reconstructed scene is visible through each of those individualobserver windows 21, while it is also thinkable that each individualobserver window 21 in the visibility region 20 shows the reconstructedscene from a different perspective.

This way, a virtual visibility region 20 can be generated which includesat least two, in this embodiment three, virtual observer windows 21 (notshown) at the same time. This substantially enlarges the virtualvisibility region 20 compared with that shown in FIG. 2, so that it isnow possible for an observer to watch the reconstructedthree-dimensional scene binocularly. It is particularly advantageousthat the modulated wave front WF is imaged into the observer plane 15 onto the observer eyes, while the Fourier transform FT is imaged on to thescreen 12. In order to achieve this, the screen 12 must be disposed inthe second focal plane of the imaging element 13. This way the periodiccontinuation of the diffraction orders is transferred to the screen 12,and displaced in particular out of the screen 12, so that only oneperiod of the diffraction spectrum is shown on the screen 12. This meansthat an observer does not perceive the periodic continuation of thereconstruction in the diffraction orders. If the wave front WF isencoded as a hologram on the light modulator device 4, the Fouriertransform FT will be imaged into the observer plane 15 and the modulatedwave front WF will be imaged on to the screen 12. In this case, ifmultiple virtual observer windows 21 are generated in the virtualvisibility region 20, the periodic continuations of the individualreconstructions will be substantially disturbing when watching thereconstructed three-dimensional scene. Further, a great advantage isthat with such a large visibility region 20 which comprises multipleobserver windows 21, successively arranged points of a reconstructedscene, i.e. points of the scene which lie in different section planes,exhibit like brightness and visibility due to the encoding. All pointshave the same brightness and are clearly perceivable or visible. It isthus prevented that one point is perceived sharp and another oneblurred, as would be the case with a single large observer window thatcorresponds with the visibility region. This makes it very difficult towatch and to perceive the reconstructed scene. Because the visibilityregion 20 according to this invention is composed of multiple smallobserver windows 21, the previously used encoding method can continue tobe used, thus avoiding the above-mentioned drawback which was caused bythe encoding.

Further, it is advantageous if the light emitted by the light sources 2falls areally on plane 11, whereby the respective virtual observerwindow 21 in the virtual visibility region 20 can be enlarged in thenon-coherent direction. This means that the wider the wave frontincident on plane 11 in the non-coherent direction, the larger is thevirtual visibility region 20 in the non-coherent direction. It is thusadvantageous if the focal length of the imaging element 9 is greaterthan the focal length of the imaging element 7, in order to affect thesize, i.e. to enlarge, the virtual visibility region 20. This would onlyrequire a single deflection element 14, or a more simple deflectionelement 14 in the holographic projection device, because the virtualvisibility region 20, which comprises multiple virtual observer windows21, must only be built up horizontally.

Diffusing elements, such as diffuser foils or similar elements, canadditionally be disposed in a plane which is projected on to the screen12, in order to enlarge the virtual observer windows 21 in thenon-coherent direction.

The observer can now move within the observer plane 15 in a very largevisibility region 20 without the need to track the observer window 21.The observer can watch the reconstructed three-dimensional scenebinocularly without any limitations from throughout this large observermobility range.

FIG. 4 shows another embodiment of the scanning system AS for theenlargement of the visibility region 20. In this embodiment, the lightsources 2 of the illumination device 1 are arranged at an angle to theoptical axis OA. The light can thus fall or be focused directly on thescanning element 3 at different angles of incidence, so that the largeimaging element L as shown in FIG. 3 can be omitted. In each lightchannel, an imaging element 5 is disposed behind the light source 2 forbroadening the light, and the imaging element 6 serves for focussing.Another difference to FIG. 3 lies in the different arrangement of thelight modulator devices 4. They are also of a reflective type, but aredisposed parallel to each other and to the optical axis OA. Generaldesign and working principle of this scanning system AS are identical tothose of the scanning system AS explained regarding FIG. 3. Design andworking principle of the projection system PS, which is disposed behindthe latter, seen in the direction of light propagation, are alsoidentical to those shown in FIG. 2, and to the one mentioned aboveregarding FIG. 3. Again, a large virtual visibility region 20 can begenerated this way in the observer plane 15, whereby the virtualobserver windows 21 are again generated using a spatial divisionmultiplexing method, as shown in FIG. 3.

Yet another possibility of enlarging the visibility region 20 in theobserver plane 15 is to combine at least two scanning systems AS. Thescanning systems AS can comprise multiple light modulator devices 4 andcan be designed for example as shown in FIGS. 3 and 4. However, it mustbe ensured that the wave fronts WF, which are modulated by the lightmodulator devices 4 and imaged into plane 11, are attached side by sideor lie next to each other or at least very close to each other. Thedifference to the two other, afore-mentioned options is that theindividual scanning systems AS are independent of each other, becausethey are not coupled electronically with each other. This boasts theadvantage that aberrations are less likely to occur, because theopenings of the scanning systems AS are sufficiently small.

Besides spatial division multiplexing, as described regarding FIGS. 3and 4, it is also possible to employ a time division multiplexing methodfor the generation of multiple virtual observer windows 21 in thevirtual visibility region 20. This is particularly advantageous becausethe number of light modulator devices 4 can thereby be substantiallyreduced to only one. With time division multiplexing it is possible touse just one light modulator device 4 if it is very fast and itsresolution is sufficiently high. The individual observer windows 21 aregenerated sequentially at a very fast pace in the observer plane 15,which results in an enlarged visibility region 20. This is why timedivision multiplexing should be preferred over spatial divisionmultiplexing, because it ensures a compact design of the entireholographic projection device, and because no additional opticalelements, such as light modulator devices, imaging elements, scanningelements etc. must be provided. Further, the holographic projectiondevice can thus be manufactured more inexpensively.

FIG. 5 illustrates the virtual visibility region 20 in detail. It is atop view which shows the screen 12 and the virtual visibility region 20,where two virtual observer windows 21 a and 21 b are generated. The twovirtual observer windows 21 a and 21 b are generated in the observerplane 15 in the virtual visibility region 20 using the spatial divisionmultiplexing method. This means that two wave fronts WF, which aremodulated by two light modulator devices 4, are imaged simultaneouslythrough imaging elements and the screen 12 into the virtual visibilityregion 20, forming there two virtual observer windows 21 a and 21 b. Thetwo wave fronts WF are represented by different line types in the Figure(dotted line and broken line). This can be achieved in two differentways. The first possibility is to encode the target wave front directlyon the light modulator device 4 and to image it into the virtualvisibility region 20. The second possibility is to start at the targetobserver windows 21, where all wave fronts of the observer windows 21are simultaneously transformed into a hologram on a light modulatordevice 4. The wave fronts are thus encoded as a hologram and generatedin the virtual visibility region 20 by way of a back-transformation onto the observer eyes.

The two virtual observer windows 21 a and 21 b are generated or formedsuch that they lie next to each other and are at least almost attachedto each other. They can also be generated such that the virtual observerwindows 21 a and 21 b are at least partly overlapped. Moreover it ispossible that there is a free space left between the generated virtualobserver windows 21 a and 21 b. As already mentioned above, the imagingof the modulated wave front WF into the virtual visibility region 20 isparticularly advantageous, because the periodic continuations in thediffraction orders, as they would occur with a reconstruction of thewave front from a hologram which is encoded on the light modulatordevice 4, can thus be prevented. If more than two observer windows 21are necessary, then more than two light modulator devices 4 must beprovided, while the observer windows 21 are generated as describedabove. It is of course alternatively possible to generate the virtualobserver windows 21 a and 21 b or more virtual observer windows 21 usingthe time division multiplexing method, where in the most favourable casejust one light modulator device 4 will be necessary. Referring to FIG.5, at first the observer window 21 a and then the observer window 2 lbare generated one after another at a very fast pace through thedeflection element 14. This must be done fast enough for an observer tonot perceive the sequential generation of the observer windows 21 a and21 b.

Using a multiplexing method, multiple virtual observer windows 21 canthus be generated in the virtual visibility region 20 in order toenlarge the virtual visibility region 20. Now, this enables an observerin the observer plane 15 to change his position and to watch thereconstructed, preferably three-dimensional scene without anyrestrictions without the need to detect the position of the observereyes in order to track the virtual observer window 21 if the observermoves. Further, even a moving scene can be represented in real timewithout complicated additional elements and methods in a simpler andfaster way, in particular if a spatial division multiplexing method isemployed.

The above-described embodiments of the holographic projection deviceonly relate to the observation of the reconstructed three-dimensionalscene by one observer. FIG. 6 shows a holographic projection device formultiple observers without the need to track the observer window 21 ifan observer moves. FIG. 6 shows a small detail of the entire holographicprojection device, namely the screen 12 and multiple observer planes150, 151, 152, 153 and 154. The number of observer planes depends on thenumber of observers and on their position in relation to the screen 12.A virtual visibility region 20 is generated in the observer plane 154,for example, as described above under FIG. 6. In order to enablemultiple observers to watch the reconstructed scene, the virtualvisibility region 20 is reproduced in the observer plane 154 and infurther observer planes 150, 151, 152 and 153. The virtual visibilityregion 20 is reproduced by at least one beam splitter element 24, sothat virtual visibility regions 200, 201, 202 and 203 are generated atthe observer positions in the respective observer planes 150, 151, 152and 153. The number of observers in the observer planes 150, 151, 152,153, 154 and so on, i.e. the number of observers watching thereconstructed scene, determines how often the virtual visibility region20 must be copied. The at least one beam splitter element 24 is disposedin front of the screen 12, seen in the direction of light propagation,in particular in front of a last imaging element, seen in the directionof light propagation. This means that the beam splitter element 24 canbe disposed either in front of the imaging element 13, or between theimaging element 13 and the screen 12, seen in the direction of lightpropagation. Other positions in the holographic projection device arepossible as well. This way, the at least one beam splitter element 24reproduces the virtual visibility region 20 so often that each observercan watch the reconstructed scene, even if he moves in the respectivevisibility region 200, 201, 202, 203 and so on. The visibility region 20is reproduced with the help of spatial division multiplexing, wherepreferably multiple beam splitter elements 24, 25 are disposed in theholographic projection device. The beam splitter elements 24, 25 can forexample be arranged in a cascading fashion for this. Mirror elements areprovided for beam guidance to the respective observer positions in theobserver planes 150, 151, 152, 153 and 154. Alternatively, thevisibility region 20 can be reproduced using time division multiplexing.Tracking will then no longer be necessary.

Further, a light modulator device with micro-mirrors as modulationelements can be used in the holographic projection device according tothis invention, because micro-mirrors are independent of each other. Thecomputational power of a computing device used can be expended undersimple conditions and using simple means. Further, already existingsoftware can be used with accordingly adapted hardware implementation.The holographic projection device with light modulator devices withmicro-mirrors, or with conventional light modulator devices can thus berealised using technologies which are already available.

Possible applications of the holographic projection device includedisplays for a two- and/or three-dimensional representation in privateor working environments, for example TV screens, computer displays,electronic games, in the entertainment industry, for example for movieprojections or events, in the automotive industry for displayinginformation, in the entertainment industry, in medical engineering, herein particular for minimally-invasive surgery applications or spatialrepresentation of tomographically established information, and inmilitary engineering for the representation of surface profiles. It willappear to those skilled in the art that the present holographicprojection device can also be applied in other areas not mentionedabove.

I claim:
 1. A method for an enlargement of a virtual visibility regionfor watching a reconstructed scene, comprising: emitting sufficientlycoherent light from at least one light source so as to generate a wavefront of the scene being encoded on at least one light modulator device;and generating a virtual visibility region for watching thereconstructed scene by imaging the wave front into an observer plane,where the virtual visibility region comprises at least two virtualobserver windows, which are dimensioned such that the reconstructedscene is watchable without tracking the observer windows if an observermoves in the observer plane, and wherein the at least two virtualobserver windows are generated in the virtual visibility region bymultiplexing in order to enlarge the virtual visibility region.
 2. Themethod according to claim 1, wherein the visibility region is reproducedusing temporal multiplexing in order to enable multiple observers towatch the reconstructed scene.
 3. The method according to claim 1,wherein in order to enable multiple observers to watch the reconstructedscene, the virtual visibility region is reproduced in at least oneobserver plane.
 4. The method according to claim 1, wherein thevisibility region is reproduced using at least one beam splitter elementin order to enable multiple observers to watch the reconstructed scene.5. The method according to claim 4, wherein the virtual visibilityregion is reproduced by at least one beam splitter element, and thatvirtual visibility regions are generated at respective observerpositions in at least one observer plane.
 6. The method according toclaim 1, wherein the at least two observer windows in the virtualvisibility region are generated using a spatial division multiplexingmethod or a time division multiplexing method.
 7. The method accordingto claim 1, wherein the at least two observer windows are generated inthe virtual visibility region such that they are at least partlyoverlapped.
 8. The method according to claim 1, wherein the at least twoobserver windows are generated in the virtual visibility region suchthat they are at least almost attached to each other.
 9. The methodaccording to claim 1, wherein a Fourier transform of the light emittedby the light source and modulated by the at least one light modulatordevice is imaged on to an imaging element which serves as a screen,where at least this imaging element images a wave front modulated bymeans of the light modulator device into the virtual visibility region.10. The method according to claim 1, wherein a colour reconstruction ofthe scene is performed simultaneously or sequentially for three primarycolours.
 11. The method according to claim 1, wherein a reconstructedthree-dimensional scene or a reconstructed moving three-dimensionalscene is displayed.
 12. The method according to claim 10, wherein thereconstructed three-dimensional scene is displayed in real time.
 13. Aholographic device for an enlargement of a visibility region forwatching a reconstructed scene comprising at least one light modulatordevice and at least one light source, where the holographic device isadapted to carry out the method according to claim 1.